1. Field of the Invention
The present invention relates to optical fibers used to transmit and/or amplify optical signals.
2. Description of the Prior Art
An optical fiber conventionally comprises a core surrounded by optical cladding and protected by a covering. The function of the core is to transmit, and possibly to amplify, an optical signal, and the function of the optical cladding is to confine the optical signal within the core. To this end, the refractive indices n1 of the core and n2 of the cladding are such that n1>n2.
Optical fibers used for transmission can have a monomode core for propagating a single optical signal or a multimode core for propagating a plurality of optical signals simultaneously. The monomode or multimode character of an optical fiber depends essentially on its core diameter. Multimode transmission fibers are advantageously used for short-haul applications at moderate bit rates, i.e. with a narrow bandwidth.
Various existing types of transmission fibers satisfy different optical property criteria such as bandwidth and attenuation, cost, fabrication constraints, etc. Depending on the applications, it may be particularly important to favor the transmission of data with a wide bandwidth (high bit rate) or to guarantee limited attenuation (long-haul transmission).
Attempts have been made to increase the bandwidth by producing graded index fibers, for example, or by producing fibers with a polymer optical cladding around a silica core, known as polymer-clad silica fibers (PCSF), although in the case of a PCSF performance is limited to around 100 MHz.km.
There are also plastics material optical fibers, i.e. fibers whose core is made from a plastics material, and which have the particular feature of a very wide bandwidth. For example, the “Lucina” fiber from Asahi Glass has a bandwidth in excess of 500 MHz.km, compared to approximately 100 MHz.km for a graded index silica fiber. However, this kind of plastics material fiber has a high attenuation, of the order of 50 dB/km to 100 dB/km, compared to 5 dB/km to 10 dB/km for a PCS fiber as previously described. Furthermore, this kind of plastics material optical fiber is costly and ages badly.
The object of the present invention is to propose a new optical fiber structure that achieves a compromise between the advantages of PCS fibers and plastics material fibers.
In particular, the bandwidth of plastics material fibers is improved by defects in the polymer constituting the multimode core of the fiber, these defects generating mode changes in the transmitted optical signals.
On the other hand, the plastics material strongly attenuates the intensity of the transmitted signal, which is unacceptable in high-power applications such as amplifying fibers or long-haul transmission fibers.
To this end the invention proposes an optical fiber comprising a double plastics material cladding surrounding a silica core. Thus mode coupling is assured by an intermediate cladding with non-homogeneous portions and whose refractive index is substantially equal to that of the core. The second optical cladding assures the conventional function of optically guiding the signal.
Moreover, so-called “cladding-pumped” optical fibers are used for amplification and generally have two concentric cores: a monomode central core consisting of an amplifier medium and intended to propagate a single optical signal to be amplified, and a multimode core around the monomode core, used to propagate a pumping wave. Optical cladding surrounds the multimode core. The refractive index is highest in the central core and lowest in the cladding. Thus the two concentric cores behave like waveguides.
An amplifier optical fiber operates on the principle known as stimulated emission, whereby a material can emit a light wave of the same wavelength and with the same phase as the transmitted light wave because of excitation of the material by means of a high-energy light source, typically a pumping laser whose wavelength is shorter than that of the signal transmitted in the fiber. The main amplifier materials used are ions of rare earths such as erbium or ytterbium, for example, and are generally integrated into the monomode central core of the fiber as dopants.
The pumping beam is conventionally emitted by an injection laser adapted to emit a high-power beam. The pumping beam propagates in the multimode core of the optical fiber and regularly passes through the monomode core. The interaction between the pump and the amplifier medium, on the one hand, and between the signal and the amplifier medium, on the other hand, amplifies the signal. The signal beam is thus amplified optically by population inversions due to absorption of the pumping beam by the amplifier material of the central core.
The efficacy of amplification is directly related to the efficacy of pumping, which is essentially conditioned by the efficacy of the coupling between the monomode core and the multimode core.
Now, one of the problems that arises with cylindrical concentric cores is that some light rays conveyed by the multimode core follow a helical trajectory around the monomode core without ever penetrating it, i.e. without ever passing through the amplifier material. The energy carried by these rays is therefore injected into the fiber in vain, because it is never used to amplify the signal transported in the monomode core.
Accordingly, pumped optical fibers generally include a particular structure which enables them to accept more optical pumping power and thereby increase the amplification of the optical signal which is obtained by absorption of energy. For example, in an attempt to solve this problem, it has already been proposed, in particular in the patent U.S. Pat. No. 5,966,491, to form longitudinal grooves in the external surface of the multimode core, in order to break up the helical trajectories of the light rays and to force them to encounter the interface of the monomode core.
Providing an irregular geometry interface between the multimode core and the optical cladding is also known in the art, as described in the patent application WO 01/38244, in order to modify the trajectories of the optical signals and cause mode changes or to introduce solid particles into the optical cladding, as described in the patent application EP 1 072 911 A1, in order to interfere mechanically with the trajectories of the light rays of signals propagating in the multimode core.
The present invention proposes another solution for increasing mode coupling in an amplifier optical fiber.